Around 1035, Peter gave up teaching to retire from the world and become a Benedictinemonk. His health suffered, especially when he tried to replace sleep with prayer. He was forced to spend time in recovery; he used it to study Scripture, and when he was healthy, he was assigned to teach his brother monks and then the public. Economus of Fonte-Avellana; prior of the house in 1043, a post in which he served for the rest of his life. He expanded the monastery, greatly improved its library, and founded sister hermitages in San Severino, Gamugno, Acerata, Murciana, San Salvatore, Sitria, and Ocri. Friend of the future PopeSaintGregory VII.

A prolific correspondent, he also wrote dozens of sermons, seven biographies (including a one of SaintRomuald), and poetry, including some of the best Latin of the time. He tried to retire to live as a simple monk, but was routinely recalled as papallegate, called upon to make peace between arguing monastic houses, clergymen, and government officials, etc. Declared a Doctor of the Church in 1828.